![]() We don’t have much control over the impedance in the driver or in the load but, we can control the impedance on the PCB. This means that wherever the trace travels, even if it changes layers, the impedance should be the same throughout the part, from the source to the destination. Well-controlled impedance means that the trace impedance is constant at every point along the path on the PCB. When routing the PCB, special attention needs to be paid to any mismatches in impedance, and efforts should be made to ensure that impedances are maintained as well as possible throughout the part of a routed signal.įor products requiring CE and EMC approval special consideration to this point is needed. So, reflection problems will be seen at that point and serious signal integrity issues can be encountered in high-speed designs. In this case, we will have some energy reflected at the transition point. When this trace enters another layer, the impedance goes up to 50 ohms, due to the geometry of the PCB. Image from Ĭonsider a trace, where the impedance measures 40 ohms. The graph on the left shows a net with potential signal integrity issues, the graph on the right is the same net with a theoretical series termination resistor of approximately 40 ohms added. The radiation can couple its energy to neighboring traces or, affect some susceptible components on the board. As a result, the miss-match of the impedance will cause some electromagnetic radiation in that localized area where this transition occurs, and where these reflections appear. Any unmatched impedances on the PCB will result in some electromagnetic interference (EMI). Rather, a distorted waveform with overshoots and undershoots and some ringing will be observed. When you look at the waveform for this, a pure square wave will not be seen. So, some of the energy of the signal will reflect toward the driver and the remaining signal will continue onward. The presence of an impedance change or discontinuity will certainly cause a reflection back to the source of that signal.Ī major issue with boards that don’t have impedance matching is the presence of reflections. It’s important to note that the voltage may vary significantly as it is propagated along the trace. Among these 3 variables, the trace width I the one that is within the complete control of the designer.Ī targeted impedance on a PCB trace can be attained by varying its width. Typically, this will be 35 or 70 microns depending on how the stack-up is defined. T is the trace height or copper thickness. H represents the distance between the trace and the adjacent plane. And, the more uniform the trace the more consistent the dielectric constant and we will achieve more consistent impedance and less power degradation.Ĭritical issues can arise if impedance matching is not performed. In other words, the shape and size should be as uniform as possible running across a consistent dielectric constant of the material that it’s running along the length for a given routing layer. Therefore, a trace that has a very uniform cross-sectional geometry needs to be constructed. Power is transmitted uniformly across the length of the trace across the PCB when there is uniform impedance. Some of the variables on which the impedance of a trace is dependent on are more or less fixed by the PCB manufacturer and some of them are defined by the PCB designer. What are some of the factors affecting impedance?
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